Hydrodynamic bearing unit

ABSTRACT

The squareness of the both end surfaces of the flange portion with respect to the outer circumference of the shaft portion of the shaft member is set to 0.001 mm or less respectively, and the flatness of both end surfaces of the flange portion is set to 0.001 mm or less respectively. The squareness of the end surface of the bearing member with respect to the inner circumference of the bearing member is set to 0.002 mm or less, and the flatness thereof is set to 0.0015 mm or less. The flatness of the inner surface of the bottom portion of the housing is set to 0.002 mm or less.

This application is a continuation application which claims the benefitof application Ser. No. 09/925,830, filed Aug. 9, 2001 now U.S. Pat. No.6,712,514. The disclosure of the prior application is herebyincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a hydrodynamic bearing unit. Thisbearing unit is preferably used particularly for supporting a spindlemotor in information apparatus, for example, in a magnetic disk devicesuch as an HDD, an FDD or the like, an optical disk device such as aCD-ROM, a DVD-ROM or the like, and a magneto-optical disk device such asan MD and an MO or the like, or for supporting a spindle in a polygonscanner such as a laser beam printer (LBP) or the like.

In the spindle motor of each kind of the above information apparatus, ahigher speed, a lower cost, and a lower noise are demanded in additionto the high rotation precision or the like. One of the key elements thatdetermine these performance requirements is a bearing for supporting aspindle of the motor. In recent years, as this kind of bearing unit, theusage of the hydrodynamic bearing with excellent characteristics isinvestigated to meet the demanded performance, or the bearing isactually used.

In spindle motors for the above information apparatus in recent years,the high rotation precision is more strongly demanded in an attempt toincrease the information recording density and obtain higher speedrotation. In order to meet this demand, still higher rotation precisionis demanded with respect to the hydrodynamic bearing incorporated in theabove spindle motors.

As a factor which largely affects rotation precision of hydrodynamicbearings, clearance control in a radial bearing clearance and a thrustbearing clearance for generating dynamic pressure is consideredimportant.

The present invention aims at providing a hydrodynamic bearing unit witha high rotation precision wherein clearance control of the radialbearing clearance and the thrust bearing clearance is made appropriate.

Furthermore, an object of the present invention is to suppress thefriction of a radial bearing surface and a thrust bearing surface tomaintain an excellent bearing performance in the hydrodynamic bearingunit described above.

SUMMARY OF THE INVENTION

The present invention provides a hydrodynamic bearing unit comprising: ashaft member having a shaft portion and a flange portion; a bearingmember arranged on an outer circumference of the shaft member; and aradial bearing portion and a thrust bearing portion respectivelycomprising bearing surfaces with grooves (dynamic pressure grooves) forgenerating hydrodynamic pressure and bearing clearances facing thebearing surfaces, and supporting the shaft member in non-contact state,in a radial direction and a thrust direction respectively, withhydrodynamic pressure generated in the bearing clearances when the shaftmember and the bearing member relatively rotate, wherein the squarenessof the both end surfaces of the flange portion with respect to the outercircumference of the shaft portion of the shaft member is 0.001 mm orless respectively, and the flatness of the both end surfaces of theflange portion is 0.001 mm or less respectively.

The “bearing member” in this invention includes a structure in which thebearing member having a radial bearing surface and a thrust bearingsurface is fixed to a housing and a structure in which a radial bearingsurface and a thrust bearing surface are directly formed on a housing.

In this case, it is desirable that the squareness of the end surface ofthe bearing member located opposite to one of the both end surfaces ofthe flange portion via the thrust bearing clearance with respect to theinner circumference of the bearing member is set to 0.002 mm or lesswhile the flatness thereof is set to 0.0015 mm or less. Furthermore, theflatness of the surface located opposite to the other end surface of theflange portion via the thrust bearing clearance is set to 0.002 mm orless.

Furthermore, according to the present invention, in a hydrodynamicbearing unit comprising: a housing with a bottom portion; a bearingmember fixed to an inner circumference of the housing; a shaft memberhaving a shaft portion inserted into the inner circumference of thebearing member and a flange portion; a radial bearing portion providedbetween the inner circumference of the bearing member and the outercircumference of the shaft portion of the shaft member for supportingthe shaft member in a radial direction in non-contact state with adynamic pressure generated in a radial bearing clearance; and a thrustbearing portion respectively provided between each end surface of theflange portion of the shaft member and an end surface of the bearingmember or an inner surface of the bottom portion of the housing forsupporting the shaft member in a thrust direction in non-contact statewith a dynamic pressure generated in the thrust bearing clearances, theflatness of the inner surface and the outer surface of the housing is0.005 mm or less.

In addition to the above structure, the squareness of the both endsurfaces of the flange portion with respect to the outer circumferenceof the shaft portion of the shaft member can be set to 0.001 mm or lesswhile the flatness of the both end surfaces of the flange portion can beset to 0.001 mm or less. Furthermore, the squareness of the end surfaceof the bearing member located opposite to one of the both end surfacesof the flange portion via the thrust bearing clearance with respect tothe inner circumferential surface of the bearing member can be set to0.002 mm or less while the flatness thereof can be set to 0.0015 mm orless. Furthermore, the flatness of the inner surface of the bottomportion of the housing can be set to 0.002 mm or less.

Furthermore, the present invention provides a hydrodynamic bearing unitcomprising: a housing with a bottom; a bearing member fixed to an innercircumference of the housing; a shaft member having a shaft portioninserted into the inner circumferential surface of the bearing memberand a thrust plate provided on the shaft portion; a radial bearingportion provided between the inner circumferential surface of thebearing member and the outer circumferential surface of the shaftportion of the shaft member for supporting the shaft portion in a radialdirection in non-contact state with a dynamic pressure action of fluidgenerated in a radial bearing clearance; and a thrust bearing portionrespectively provided between each end surface of the thrust plate ofthe shaft member and the lower end surface of the bearing member or thebottom surface of the housing for supporting the thrust plate in athrust direction in non-contact state with a dynamic pressure action offluid generated in a thrust bearing clearance; wherein the surfacehardness of the outer circumferential surface of the shaft portion islarger than that of the inner circumferential surface of the bearingmember, the surface hardness of the both end surfaces of the thrustplate is larger than that of the lower end surface of the bearing memberand the bottom surface of the housing, the surface roughness of theouter circumferential surface of the shaft portion is smaller than thatof the inner circumferential surface of the bearing member, the surfaceroughness of the both end surfaces of the thrust plate is smaller thanthat of the lower end surface of the bearing member and the bottomsurface of the housing, and the outer circumferential surface of theshaft portion has a surface characteristic on which fine projectionsconstituting the surface roughness is smoothed. The surface with suchsurface characteristic can be formed through grinding process or thelike followed by tumbler process, barrel process or the like.Alternatively the surface can be formed through grinding process or thelike followed by relatively sliding process with the slide member with asurface hardness larger than the surface.

In the above structure, preferably, the outer circumferential surface ofthe shaft portion has a mean square inclination angle Δq, designated inISO4287/1, of 2.0 or less.

In the above structure, the both end surfaces of the thrust plate canassume a structure in which the both end surfaces of the thrust platehas a surface characteristic on which fine projections constituting theroughness of the surface is smoothed. In this case, the both endsurfaces of the thrust plate can have the mean square inclination angleΔq, designated in ISO4287/1, of 2.0 or less.

Furthermore, in the above structure, preferably the outercircumferential surface of the shaft portion has the arithmetic averagedeviation Ra, designated in ISO4287/1, of 0.04 μm or less, while theboth end surfaces of the thrust plate have the arithmetic averagedeviation Ra, ruled out in ISO4287/1, of 0.04 μm or less, preferably Ra0.01 μm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a spindle motor having a hydrodynamicbearing unit according to the present invention.

FIG. 2 is a sectional view showing a hydrodynamic bearing unit accordingto the first embodiment of the present invention.

FIG. 3 is a view showing a relation between the squareness of the outercircumference of the shaft portion with respect to the end surface ofthe flange portion and radial NRRO.

FIG. 4 is a sectional view showing a hydrodynamic bearing unit accordingto the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a view showing one example of a structure of a spindle motorfor an information apparatus incorporating a hydrodynamic bearing unit 1according to this embodiment. This spindle motor is used in a disk drivedevice such as an HDD or the like. The spindle motor comprises thehydrodynamic bearing unit 1 supporting a shaft member 3 such that it canrotate freely in non-contact state, a disk hub 4 attached on then shaftmember 3, a motor stator Ms, and a motor rotor Mr. The stator Ms and therotor Mr are located opposite position via a gap in a radial direction.The stator Ms is attached on the outer circumferential surface of thecasing 6 while the rotor Mr is attached on the inner circumferentialsurface of the disk hub 4. The housing 5 of the hydrodynamic bearingunit 1 is attached on the inner circumferential surface of the casing 6.On the disk hub 4, one or a plurality of disks D such as magnetic disksor the like is/are held. When the stator Ms is electrified, the rotor Mris rotated with the excitation force between the stator Ms and the rotorMr, so that the disk hub 4 and the shaft member 3 are integrallyrotated.

FIG. 2 is a view showing the first embodiment of the hydrodynamicbearing unit 1. The bearing unit 1 comprises a shaft member 3, acylindrical housing 5 with a bottom, a bearing member 7 and a sealingmember 9 such as a seal washer or the like for sealing one end side ofthe bearing member 7 (open side of the housing 5) as main constituentelements. The shaft member 3 has a shaft portion 3 a, and a flangeportion (a thrust plate) 3 b provided on one end portion of the shaftportion 3 a. The shaft portion 3 a is arranged on an innercircumferential surface of the bearing member 7 and the flange portion 3b is arranged between an end surface 7 b of the bearing member 7 and abottom portion 51 of the housing 5. The bottom portion 51 of the housing5 closes the open portion on one end of the housing 5, and is integrallyformed on the housing 5, or may be formed of a separate lid member.

The bearing member 7 is formed of soft metal or sintered metalimpregnated with oil or the like. On the inner circumference of thebearing member 7, a radial bearing surface 11 a with a plurality ofdynamic pressure grooves by transferring, rolling or the like in pressprocess. As a consequence, at the time of the relative rotation of theshaft member 3 and the bearing member 7 (at the time of the rotation ofthe shaft member 3 in the embodiment), a dynamic pressure of the fluid(for example, lubricating oil) filled in a radial bearing clearance Crbetween the radial bearing surface 11 a and the outer circumferentialsurface of the shaft portion 3 a is generated. This dynamic pressureaction constitutes a radial bearing portion 11 for supporting the shaftportion 3 a in a radial direction in non-contact state.

On the both sides of the flange portion 3 b, thrust bearing clearancesCs1 and Cs2 which are clearances in an axial direction are provided. Thethrust bearing clearance Cs1 is formed between one end surface 3 b 1 ofthe flange portion 3 b and the end surface 7 b of the bearing member 7located opposite thereto while the other thrust bearing clearance Cs2 isformed between the other end surface 3 b 2 of the flange portion 3 b andthe inner surface 51 a of the bottom portion 51 of the housing 5. On endsurfaces facing the thrust bearing clearances Cs1 and Cs2, for example,on the both end surfaces 3 b 1 and 3 b 2 of the flange portion 3 b,thrust bearing surfaces 13 a and 13 b with dynamic pressure grooves forthe generation of the hydrodynamic pressure are formed. The dynamicpressure of the fluid is generated in the thrust bearing clearances Cs1and Cs2 at the time of the above rotation, so that a thrust bearingportion 13 is constituted for supporting the flange portion 3 b fromboth sides in a thrust direction in non-contact state.

The configuration of the dynamic pressure grooves of the radial bearingsurface 11 a and the thrust bearing surfaces 13 a and 13 b can bearbitrarily selected. Any of the known herringbone type, spiral type,step type, multiple arc type or the like can be selected, or any ofthese types can be appropriately combined to be used.

By the way, an HDD, one kind of information apparatus, having tens ofthousands of tpi (Track per inch) is being developed for a largercapacity. For example, in the case that an HDD with 20,000 tpi has adistance of 1.27 μm between tracks, about one tenth or less thereof(0.13 μm or less) is demanded as a radial non-repeated run out (NRRO) ofthe spindle motor.

At present, an HDD with 50,000 tpi are put into practical use where 0.05μm or less is demanded as NRRO. FIG. 3 is a view showing the measurementresult of the change in the radial NRRO in the case that the squarenessof the outer circumference of the shaft portion 3 a, and the end surfaceof the flange portion 3 b is made different. According to FIG. 2, inorder to suppress the NRRO to 0.05 μm or less, it can be understood that1 μm or less is required as the squareness. Consequently, it is desiredthat the squareness of the both end surfaces 3 b 1 and 3 b 2 of theflange portion 3 b with respect to the outer circumference of the shaftportion 3 a of the shaft member 3 is set to 0.001 mm or lessrespectively (desirably, 0.0005 mm or less).

It is difficult to control the thrust bearing clearance Cs1 to anappropriate value only by regulating the squareness. From thisviewpoint, it is desirable to set the flatness of the both end surfaces3 b 1 and 3 b 2 of the flange portion 3 b to 0.001 mm or less(desirably, 0.0005 mm or less).

Here, the “squareness” means, in a combination of a predetermined planarsurface with a reference surface, a deviation scale on the abovepredetermined surface from a geometric surface in geometrically rightangle with respect to the reference surface. This is represented, forexample, by measuring the amplitude (maximum value) of the both endsurfaces 3 b 1 and 3 b 2 by contacting respective terminals to the bothend surfaces 3 b 1 and 3 b 2 of the flange portion while rotating theshaft member 3 on an axis center. Furthermore, the “flatness” means aheight difference between the maximum convex portion and minimum concaveportion on a measurement surface. In the case where dynamic pressuregrooves are present on a planar surface which becomes an object in anycase, an imaginary surface connecting ridges (projections) between thedynamic pressure grooves is formed as a reference (the same holds trueof the following).

The squareness of the end surfaces 7 a of the bearing member 7 locatedopposite to one end surface 3 b 1 of the both end surfaces 3 b 1 and 3 b2 of the flange portion 3 b with respect to the inner circumference ofthe bearing member 7 is set to 0.002 mm or less (preferably, 0.0015 mmor less) while the flatness of the end surface 7 a is set to 0.0015 mmor less (desirably, 0.001 mm or less).

Furthermore, the flatness of the inner surface 51 a of the bottomportion 51 located opposite to the other end surface 3 b 2 of the flangeportion 3 b via the thrust bearing clearance Cs2 is set to 0.002 mm orless (desirably, 0.0015 mm or less).

The radial bearing clearance Cr and the thrust bearing clearances Cs1and Cs2 can be secured to an appropriate value by regulating thesquareness and the flatness as described above, so that the contact ofthe shaft portion 3 a with the bearing member 7 or the bearing member 7and the bottom portion 51 with the flange portion 3 b in rotating isprevented, and a sufficient dynamic pressure can be generated forsupporting the shaft in each bearing clearance, thereby a high rotationprecision is attained.

Furthermore, it is possible to secure the assemblage precision at thetime of the assemblage of the bearing unit 1 by setting the parallelismbetween the inner surface 51 a and the outer surface 51 b of the bottomportion 51 to 0.005 mm or less (desirably, 0.003 mm or less).

Here, the “parallelism” means, assuming one of two planar surfaces to beparallel each other as a reference surface, a deviation scale of theother surface from a geometric surface geometrically parallel to thereference surface.

As described above, the radial bearing clearance and the thrust bearingclearances are secured to an appropriate value by controlling thesquareness and the flatness of each portion within a predetermined valueso that an unstable rotation resulting from the mutual contact of thebearing surfaces and the shortage of the dynamic pressure in the bearingclearances is prevented, thereby it is possible to suppress torque lossand torque change to obtain a high rotation precision.

Furthermore, the assemblage precision at the time of assemblage can besecured by controlling the parallelism of the inner surface and theouter surface of the housing to a predetermined value.

FIG. 4 is a view showing the second embodiment of the hydrodynamicbearing unit 1. The hydrodynamic bearing unit 1 comprises a housing 5with a bottom having a cylindrical inner circumferential surface 5 a, acylindrical bearing member 7 fixed to the inner circumferential surface5 a of the housing 5, a shaft member 3, and a sealing member 9 forshielding the upper end surface side (open side of the housing 5) of thebearing member 7 as its main constituent elements.

The housing 5 is formed of, for example, brass and comprises acylindrical side portion 5 b and a bottom portion 51. In thisembodiment, the side portion 5 b and the bottom portion 51 of thehousing 5 are integrally constituted, but they may be constituted in aseparate structure.

The shaft member 3 is formed of, for example, stainless steel (SUS420J2)or the like, and comprises the shaft portion 3 a, and the flange portion3 b (a thrust plate) integrally or separately provided on the shaftportion 3 a. The shaft portion 3 a is, inserted into the innercircumferential surface 7 a of the bearing member 7 with a predeterminedradial bearing clearance Cr, and the thrust plate 3 b is accommodatedinto the space portion between the lower end surface 7 b of the bearingmember 7 and the bottom surface 7 c of the housing 5. Thrust bearingclearances Cs1 and Cs2 are respectively provided between the upper endsurface 3 b 1 of the thrust plate 3 b and the lower end surface 7 b ofthe bearing member 7 and between the lower end surface 3 b 2 of thethrust plate 3 b and the bottom surface 51 a of the housing 5.

The bearing member 7 is formed of, for example, a porous material, inparticular, sintered metal of copper and iron. Lubricating oil orlubricating grease is impregnated into the inside pores to provideimpregnated oil bearing. Plural dynamic pressure grooves are formed in aregion constituting a radial bearing surface of the innercircumferential surface 7 a of the bearing member 7. When the bearingmember 2 is rotated, a dynamic pressure action is generated in theradial bearing clearance Cr, so that the shaft portion 3 a of the shaftmember 3 is supported in a radial direction so as to rotate freely innon-contact state, with the oil film of the lubrication oil formed inthe radial bearing clearance Cr. As a consequence, a radial bearingportion 11 is constituted for supporting the shaft member 3 in a radialdirection such that it can rotate freely in non-contact state. Thedynamic pressure groove may be formed on the outer circumference of theshaft portion 3 a of the shaft member 3.

A dynamic pressure groove is formed respectively on regions of the upperend surface 3 b 1 of the thrust plate 3 b or the lower end surface 7 bof the bearing member 7, and the lower end surface 3 b 2 of the thrustplate 3 b or the bottom surface 51 a of the housing 5, which become thethrust bearing surfaces. When the shaft member 3 is rotated, a dynamicpressure action is generated in the thrust bearing clearance Cs1 andCs2, so that the thrust plate 3 b of the shaft member 3 is supported ina thrust direction so as to rotate freely in non-contact state, with theoil film of the lubricating oil formed in the thrust bearing clearanceCs1 and Cs2, As a consequence, the thrust bearing portion 13 isconstituted for supporting the shaft member 3 in a thrust direction suchthat it can rotate in non-contact state.

The configuration of the dynamic pressure groove of the radial bearingsurface and the thrust bearing surface can be arbitrarily selected, sothat any of the known herringbone type, spiral type, step type, andmultiple arc type is selected or such types are appropriately combinedand used.

In the above structure, the surface hardness of the outer circumferencesurface of the shaft portion 3 a is larger than that of the innercircumferential surface 7 a of the bearing member 7 while the surfacehardness of the both end surfaces 3 b 1 and 3 b 2 of the thrust plate 3b is larger than that of the lower end surface 7 b of the bearing member7 and the bottom surface 51 a of the housing 5. For example, the shaftmember 3 is subjected to surface hardening process such as platingprocess, carbonation, nitration, carbo-nitration or other thermalprocess, so that the surface hardness of the outer circumferentialsurface of the shaft portion 3 a and the both end surfaces 3 b 1 and 3 b2 of the thrust plate 3 b is adjusted to Vickers hardness HV of 500 ormore, preferably about HV of 500 through 550.

Furthermore, the surface roughness of the outer circumferential surfaceof the shaft portion 3 a is smaller than that of the innercircumferential surface 7 a of the bearing member 7 while the surfaceroughness of the both end surfaces 3 b 1 and 3 b 2 of the thrust plate 3b is smaller than that of the lower end surface 7 b of the bearingmember 7 and the bottom surface 51 a of the housing 5. In thisembodiment, the outer circumferential surface of the shaft portion 3 aand the both end surfaces 3 b 1 and 3 b 2 of the thrust plate 3 b arefinished with cutting process such as grinding process or the like aftersurface hardening process, and thereafter tumbler process or barrelprocess is conducted, so that fine projections (fine projectionsconstituting the roughness of these surfaces) on the outercircumferential surface of the shaft portion 3 a and the both endsurfaces 3 b 1 and 3 b 2 of the thrust plate 3 b are smoothed. As aconsequence, in this embodiment, each mean square inclination angle Δq,designated in ISO4287/1, of the outer circumferential surface of theshaft portion 3 a and the both end surfaces 3 b 1 and 3 b 2 of thethrust plate 3 b is set to 2.0 or less. Furthermore, in this embodiment,the arithmetic average deviation Ra, designated in ISO4287/1, of theouter circumferential surface of the shaft portion 3 a is set to 0.04 μmor less while the arithmetic average deviation Ra of the both endsurfaces 3 b 1 and 3 b 2 of the thrust plate 3 b is 0.04 μm or less,preferably 0.01 μm.

The smoothing process of the fine projections is conducted only on theouter circumferential surface of the shaft portion 3 a while the bothend surfaces 3 b 1 and 3 b 2 of the thrust plate 3 b may be subjected tocutting process such as grinding process or the like. In that case, forexample, when the both end surfaces 3 b 1 and 3 b 2 of the thrust plate3 b are subjected to cutting process, the outer circumferential surfaceof the shaft portion 3 a of the shaft member 3 is supported with a shoewhose surface hardness is larger than that of the surface (for example,a shoe formed of hard material such as superhard alloy, combacs or thelike), and the shaft portion 3 a is relatively slided with respect tothe shoe, so that the surface projection of the outer circumferentialsurface of the shaft portion 3 a can be smoothed. Furthermore, in thiscase, it is preferable that the surface roughness of the both endsurfaces 3 b 1 and 3 b 2 of the thrust plate 3 b are made smaller thanthat of the outer circumferential surface of the shaft portion 3 a, and,for example, the arithmetic average deviation Ra is set to 0.0 μm orless.

As described above, according to the present invention, friction on theradial bearing surface constituting the radial bearing portion and thethrust bearing surface constituting the thrust bearing portion issuppressed, so that the excellent bearing performance of thehydrodynamic bearing unit of this type can be maintained over a longperiod.

1. A hydrodynamic bearing unit comprising a shaft member having a shaftportion and a flange portion, a bearing member arranged on an outercircumference of the shaft member; and a radial bearing portion and athrust bearing portion respectively comprising bearing surfaces withgrooves for generating hydrodynamic pressure and bearing clearancesfacing the bearing surfaces, and supporting the shaft member innon-contact state, in the radial direction and the thrust directionrespectively, with hydrodynamic pressure generated in the bearingclearances when the shaft member and the bearing member relativelyrotate; wherein the squareness of the both end surfaces of the flangeportion with respect to the outer circumference of the shaft portion ofthe shaft member is 0.001 mm or less respectively, and the flatness ofboth end surfaces of the flange portion is 0.001 mm or lessrespectively.
 2. The hydrodynamic bearing unit according to claim 1,wherein the squareness of the end surface of the bearing member locatedopposite to one of the both end surfaces of the flange portion via thethrust bearing clearance with respect to the inner circumference of thebearing member is 0.002 mm or less and the flatness thereof is 0.0015 mmor less.
 3. The hydrodynamic bearing unit according to claim 2, whereinthe flatness of a surface located opposite to the other end surface ofthe flange portion via the thrust bearing clearance is 0.002 mm or less.